1. Field of the Invention
The present invention relates, generally, to friction devices, such as clutch or brake assemblies for use in transmissions, differentials, or brake systems. More specifically, the present invention relates to multi-disk clutch or brake assemblies having low-drag characteristics.
2. Description of the Related Art
Multi-disk friction devices are employed in a wide range of applications as clutch or brake assemblies. For example, such devices are frequently used in land-based vehicles. Generally speaking, these vehicles require three basic componentsxe2x80x94a power plant (such as an internal-combustion engine), a powertrain, and wheels. The powertrain""s main component is typically referred to as a xe2x80x9ctransmission.xe2x80x9d Engine torque and speed are converted in the transmission in accordance with the tractive-power demand of a vehicle. The transmission includes one or more gear sets, which may include an inner sun-gear, intermediate planet gears that are supported by their carriers, and outer ring gears. Various components of the gear sets are held or powered to change gear ratios in the transmission. A multi-disk pack clutch is a friction device that is commonly employed as a holding mechanism in a transmission or a differential. In addition, multi-disk friction devices also find use in industrial applications, such as in wet-brake assemblies, to brake the wheels on earth-moving equipment.
The multi-disk pack clutch or the multi-disk brake assembly has a clutch sub-assembly that includes a set of plates and a set of friction disks. The plates are interleaved with the friction disks. The plates and the friction disks are bathed in a continual flow of lubricant and, in xe2x80x9copen packxe2x80x9d operation, normally turn past one another without contact. The pack clutch or the brake assembly also typically includes a piston. When a component of a gear set is to be held, as, for example, during a particular gear range, a piston is actuated to cause the plates to come into contact with the friction disks. In certain applications, it is known to employ several multi-disk pack devices in combination to establish different drive connections throughout the transmission or differential to provide various gear ratios in operation or to brake a component.
When the friction disks are not engaged, there often remains a differential rotational speed between a drive member and a driven member that the clutch or the brake bridges. Relative contact between the plates and the friction disks during openpack mode creates drag. This condition reduces the efficiency of the transmission and, ultimately, results in lower fuel efficiency.
Accordingly, there remains a need in the art for a friction device that may be selectively engaged and disengaged and ensures separation between the plates and the friction disks when the clutch is in open-pack mode to reduce drag and improve fuel economy.
The present invention overcomes the disadvantages in the related art in a friction device that includes a drive member having a drive hub and a driven member having a driven hub. The drive and driven members are rotatable about a common axis and define a housing therebetween. A clutch pack is interposed between the drive and driven members, operable to connect and disconnect the drive and driven members for transferring and interrupting torque therebetween, and has aplurality of separation plates disposed on and rotatable with the drive hub and a plurality of friction plates disposed on and rotatable with the driven hub such that the plurality of separation plates is interleaved with the plurality of friction plates. A piston assembly is supported in the housing, defines an expandable chamber between the piston assembly and the housing, and is adapted to receive fluid under pressure.
The piston assembly is responsive to pressurized fluid in the expandable chamber to move among a disengaged position wherein the plurality of friction plates are disposed in non-contacting relationship with the plurality of separation plates, a pre-staged position wherein the plurality of friction plates are disposed in proximate contacting relationship with the plurality of separation plates, and an engaged position wherein the piston assembly is disposed to force the plurality of friction plates into contacting relationship with the plurality of separation plates so as to actuate the clutch pack to connect the drive and driven members. The clutch pack further has a separation device disposed between adjacent friction plates and adapted to bias the adjacent friction plates in opposite directions to facilitate non-contacting rotation of the adjacent friction plates when torque translation between the drive and driven members is interrupted.
In this way, the friction device of the present invention is adapted to use a long-travel arrangement of the piston assembly to provide an increased amount of running clearance when the friction device is in the low-drag mode. Also, prior to engagement, the piston assembly is adapted to be pre-staged to a position that allows a more normal running clearance so that the piston assembly is engagement-ready. Furthermore, the friction device is adapted to use positive mechanical separation of the separation and friction plates to ensure an air gap among all friction surfaces while the friction device is in the low-drag mode.
One advantage of the friction device of the present invention is the elimination or minimization of parasitic spin losses in a clutch assembly.
Another advantage of the friction device of the present invention is the reduction of drag in an automatic transmission incorporating the friction device.
Another advantage of the friction device of the present invention is the improvement in fuel economy of a vehicle incorporating the friction device.
Finally, these advantages are achieved in a friction device, such as a clutch assembly or a brake assembly, that is operationally efficient and cost-effective to manufacture relative to the complex systems proposed in the related art.